As is known in the art, a Frequency Selective Limiter (FSL) is a nonlinear passive device that provides greater attenuation to signals above a predetermined threshold power level while signals below the threshold power level are not substantially attenuated. One feature of the FSL is the frequency selective nature of the high-power limiting: low power signals close in frequency to the limited signals are substantially unaffected (i.e., the FSL does not substantially attenuate such signals).
A typical implementation of an FSL includes a stripline transmission structure using two layers of dielectric material disposed about the stripline, with the stripline having a fixed length and a fixed width along the length of the FSL. Such structures are relatively simple to fabricate and provide adequate magnetic fields to realize a critical power level of approximately 0 dBm when using a single crystal material. One method of reducing the threshold power level is to use a lower-impedance stripline at the cost of degraded return loss. An external matching structure can be used to improve the impedance match, but this technique reduces the bandwidth and increases the insertion loss of the FSL.
Another approach to reducing the threshold power level is using a transmission line structure having a tapered width. M. Morton and G. Sollner in U.S. patent application Ser. No. 15/627,913 describe a frequency selective limiter (FSL) comprising a transmission line structure having a tapered width. The transmission line structure can be provided such that one or more conductors have a first width at a first end (e.g., an input) of the FSL and a second, different width at a second end (e.g., an output) of the FSL. Use of a tapered width transmission line results in a reduced insertion loss for a given threshold power without changing the structure of the magnetic material (e.g., ferrite material) disposed adjacent to the transmission line. However, this tapered transmission line structure supports narrower bandwidth than desired.